Classical Argument # 4 - The Ontological Argument

The last major classical argument that I'm going to look at is the Ontological Argument.  The word ontology refers to the study of being or existence.  This argument was first proposed by Anselm of Canterbury in 1078 in a written prayer called "Proslogion."  Anselm defined God as "that than which nothing greater can be conceived." The argument goes if God did not exist then it would be possible to conceive of something greater which is a contradiction and therefore God must exist.  Most people think this is some sort of trick with word play, but interestingly almost every major thinker in the history of philosophy has dealt with this argument.  While Anselm's version has largely been abandoned, some modern versions have resurrected the argument and today it is not only alive but surprisingly flourishing. 
For this argument, we are defining God as the "greatest possible being."  As the greatest possible being, God is by definition a necessary being.  A necessary being is by definition a being that must exists if its existence is possible.

#1 If it is possible that God exists, then God exists

#2 It is possible that God exists

#3 Therefore, God exists

Yet again, the argument is valid, meaning the conclusion follows from the premises.  The only way to argue against the conclusion is to challenge the premises.  Almost everyone, including most atheists, would agree with premise #2 (at least initially).  So the whole argument falls on premise #1.  But as stated above, if God did exist He would be the greatest possible being, and the greatest possible being would have the attribute of necessity, and something that is necessary exists if it's existence is possible.  "Yah right!" is what you are probably thinking, but it is interesting that this is relatively uncontroversial because one definition simply leads to the next.  But if premise #1 is true, then the only way to deny the argument is to go back and deny premise #2.  An opponent of the argument must show that the concept of God is incoherent or otherwise impossible. Otherwise, the argument is sound and the conclusion is true. 

Strengths:

• Most skeptics would say that God probably does not exist.  But to say that God probably does not exist is just to say that God possibly exists and therefore concedes premise #2.
• The argument makes the issue black or white - God's existence is either true or impossible. 
• If successful, the argument poses a few interesting situations.  For instance every agnostic alive would have to believe that God exists in order to be consistent with agnosticism, thereby contradicting himself.

Weaknesses:

• The argument is very abstract (and silly to some) and consequently hard to take seriously.
• It is easy to deny premise #2, which is what most opponents of the argument do.
• The argument does not argue for Christianity (as revealed by Jesus), but only for the greatest possible being.

Bible References:

• Psalm 14:1, 53:1, 145:3

If successful this argument leaves you with the greatest possible being.  This greatest possible being would have every great-making property.